The present invention relates to a sheet-fed offset printing press with a convertible press mechanism, which has a convertible cylinder disposed between upstream and downstream paper convey cylinders with respect to a paper convey direction to enable one-sided printing and double-sided printing with one printing press.
Along with a variety of printing techniques, a sheet-fed offset printing press with a convertible press mechanism, which is capable of performing one-sided and double-sided printing has been proposed and put into practice. As an example of such a offset printing press, a sheet-fed offset printing press with a convertible press mechanism, which has upstream and downstream impression cylinders with respect to a paper convey direction, and a convertible cylinder disposed between these two cylinders, is disclosed in Japanese Utility Model Laid-Open No. 7-35041. This printing press will be described.
FIG. 7 shows the cylinder arrangement of the conventional sheet-fed offset printing press with the convertible press mechanism disclosed in this reference, which is set in the double-sided printing state. Referring to FIG. 7, in each of adjacent printing units, e.g., of a first printing unit 1 and a second printing unit 2, a plate cylinder 3 on which a plate is mounted and a blanket cylinder 4 on which a blanket is wound are arranged to be in contact with each other. Impression cylinders 5 and 6 each having a diameter twice that of the plate cylinder 3 are respectively arranged to be in contact with the blanket cylinders 4 of the upstream and downstream printing units 1 and 2 with respect to the paper convey direction. A convertible cylinder 7 having a diameter twice that of the plate cylinder 3 is arranged between the impression cylinder 5 of the printing unit 1 and the impression cylinder 6 of the printing unit 2, such that its circumferential surface is in contact with the impression cylinders 5 and 6.
A plurality of sets of gripper units 8 each consisting of a gripper and a gripper pad and opened/closed by a cam mechanism are arranged at portions that equally halve the outer circumferential portion of the impression cylinder 5 in the circumferential direction, to be aligned in the axial direction of the impression cylinder 5. A plurality of sets of gripper units 9 each opened/closed by a cam mechanism are arranged at portions that equally halve the outer circumferential portion of the impression cylinder 6 in the circumferential direction, to be aligned in the axial direction of the impression cylinder 6.
A pair of converting grippers 10 each opened/closed by a cam mechanism and a pair of suckers 11 connected to a suction air source, e.g., a pump (not shown), are arranged on the outer circumferential portion of the convertible cylinder 7. The corresponding converting gripper 10 and sucker 11 form a pair, and the respective pairs of converting grippers 10 and suckers 11 are arranged at portions that equally halve the outer circumferential portion of the convertible cylinder 7 in the circumferential direction, to come close to each other and be aligned in the axial direction of the convertible cylinder 7.
When one-sided printing is to be performed, the phases of the respective cylinders are set such that, upon rotation of the cylinders 5, 6, and 7, the converting grippers 10 oppose the gripper units 8 and the gripper units 9. When double-sided printing is to be performed, as in FIG. 7, the phases of the respective cylinders are set such that, upon rotation of the cylinders 5, 6, and 7, the paper trailing end of a paper sheet 12 gripped by the gripper units 8 corresponds to the suckers 11 and the converting grippers 10 correspond to the gripper units 9.
The operation of the sheet-fed offset printing press having the above arrangement will be described with reference to FIG. 8. In the case of double-sided printing, when the cylinders 5, 6, and 7 are rotated, the upper surface of the paper sheet 12 gripped by the gripper units 8 and conveyed is subjected to printing as the paper sheet 12 passes between the blanket cylinder 4 and the impression cylinder 5. The paper sheet 12, the upper surface of which has been subjected to printing, passes between the cylinders 5 and 7 without being gripped by the converting grippers 10 at its gripper end, and is wound on the lower circumferential surface of the impression cylinder 5.
When the paper trailing end of the paper sheet 12 wound on the impression cylinder 5 reaches the contact point between the cylinders 5 and 7, that is, when the corresponding suckers 11 reach the line connecting the center of the impression cylinder 5 and the center of the convertible cylinder 7, the paper trailing end of the paper sheet 12 is drawn by these suckers 11. While the paper trailing end of the paper sheet 12 is drawn by the suckers 11, when the convertible cylinder 7 continues rotation, the suckers 11 move round along the outer side of the circumferential surface of the convertible cylinder 7 while rotating counterclockwise, as indicated by reference numerals 11A to 11G in FIG. 8.
When the suckers 11 hold the paper trailing end and move to the position indicated by the reference numeral 11G in this manner, the paper sheet 12 is turned. At this time, until the suckers 11 are located at the contact point between the cylinders 5 and 7 and draws the paper trailing end of the paper sheet 12, the gripper units 8 of the impression cylinder 5 are kept closed and do not release the paper sheet 12. Thereafter, the suckers 11 draw the paper trailing end of the paper sheet 12, and the gripper units 8 are opened simultaneously to release the paper sheet 12.
When the suckers 11 that draw the paper sheet 12 reach the position indicated by reference numeral 11G, air suction from the suckers 11 is interrupted due to the action of the rotary valve, and the paper sheet 12 is released. Simultaneously, the paper sheet 12 released from the suckers 11 is gripped by the gripping surfaces of the converting grippers 10.
When the convertible cylinder 7 is further pivoted and the converting grippers 10 oppose the corresponding gripper units 9 at the position of the contact point between the convertible cylinder 7 and impression cylinder 6, the paper sheet 12 is transferred from the converting grippers 10 of the convertible cylinder 7 to the corresponding gripper units 9 of the impression cylinder 6. Thereafter, the paper sheet 12 is gripped by the gripper units 9 of the impression cylinder 6, and its lower surface is subjected to printing while the paper sheet 12 is being conveyed.
After the paper sheet 12 is transferred to the converting grippers 10 at the position indicated by the reference numeral 11G, the suckers 11 pivot through substantially 90.degree. until they reach the contact point between the cylinders 7 and 6, and move in the radial direction to retract in the circumferential surface of the convertible cylinder 7. Therefore, the suckers 11 will not interfere with the circumferential surface of the impression cylinder 6.
When the convertible cylinder 7 is further rotated, the suckers 11 move from the contact point between the cylinders 6 and 7 to the contact point between the cylinders 7 and 5. During this period of time, the suckers 11 move in the circumferential direction of the convertible cylinder 7 while they are pivot by a sucker pivoting cam (not shown) through substantially 360.degree.. Because of the action of a sucker moving cam, the suckers 11 ride over the corresponding converting grippers 10, and move in the radial direction of the convertible cylinder 7 to project from and retract in the circumferential surface of the convertible cylinder 7.
When performing a switching operation from double-sided printing to one-sided printing, the phases of the upstream cylinders are adjusted with respect to the paper convey direction including the impression cylinder 5, such that the gripper units 8 of the impression cylinder 5 correspond to the converting grippers 10 of the convertible cylinder 7. In one-sided printing, when the cylinders 5, 6, 7, and the like are rotated, the paper sheet 12 which is conveyed as it is gripped by the gripper units 8 of the impression cylinder 5 is subjected to first-color printing while it passes through the contact point between the blanket cylinders 4 and impression cylinder 5 of the printing unit 1.
Thereafter, the paper sheet 12 is transferred from the gripper units 8 to the converting grippers 10, and is conveyed as it is wound on the lower circumferential surface of the convertible cylinder 7. The paper sheet 12 wound on the convertible cylinder 7 is transferred from the converting grippers 10 to the gripper units 9 of the impression cylinder 6 and is conveyed. When the paper sheet 12 passes through the contact point between the blanket cylinder 4 and impression cylinder 6 of the printing unit 1, the surface of the paper sheet 12 which has been printed by the printing unit 1 is subjected to second-color printing by the printing unit 2.
The operation described above is described in detail in E.P. 0641651A1.
In the conventional sheet-fed web offset printing press with the convertible press mechanism described above, in double-sided printing, to separate the paper sheet 12 wound on the circumferential surface of the upstream impression cylinder 5, its paper trailing end is separated by rotation of the convertible cylinder 7 and suction by the suckers 11. Since the gripping force effected by suction of the suckers 11 is limited as compared to the gripping force effected by the grippers, a sufficient gripping operation cannot be performed.
The paper sheet 12 which has passed through the contact point between the impression cylinder 5 and blanket cylinder 4 is in tight contact with the circumferential surface of the impression cylinder 5 because of the printing pressure applied between the impression cylinder 5 and blanket cylinder 4. The tight contact force applied to the circumferential surface of the impression cylinder 5 becomes particularly strong when performing printing on a coated printing paper sheet and when performing offset printing that uses dampening water. As a result, sometimes a so-called gripping failure occurs in which the paper trailing end of the paper sheet 12 cannot be gripped by the suckers 11 or even if it is gripped once, it is undesirably released.
In this case, when the paper sheet 12 which is failed to be gripped stays wound on the upstream impression cylinder 5, the next paper sheet lies on it, causing defective printing. When the paper sheet 12 which is failed to be gripped drops in the printing press, the dropped paper sheet 12 is caught by another roller or cylinder to cause a printing trouble. This tendency is becoming more and more conspicuous as the operation speed of the printing press increases.